Successful treatment of a unique case of solitary primary iliopsoas abscess caused by Streptococcus dysgalactiae subspecies equisimilis: A case report

Rationale: Iliopsoas abscess, mainly caused by Staphylococcus aureus, occurs via the bloodstream or spread from adjacent infected organs. Although a few cases regarding primary iliopsoas abscess caused by Streptococcus dysgalactiae subspecies equisimilis (SDSE) with accompanying disseminated foci have been reported to date, there has been no case report on solitary primary iliopsoas abscess caused by SDSE. Patient concerns: An 85-year-old Japanese woman presented with worsening right hip pain and fever after an exercise. Hip computed tomography revealed a right iliopsoas abscess (iliac fossa abscess), and intravenous cefazolin was started as a treatment based on the creatinine clearance level on admission. Diagnoses: Blood cultures were positive for β-hemolytic Lancefield group G gram-positive cocci arranged in long chains, which were identified as SDSE by matrix-assisted laser desorption/ionization. No other disseminated foci were found upon performing whole computed tomography and transthoracic echocardiography. The patient was diagnosed with an SDSE solitary iliopsoas abscess. Interventions: The antimicrobial was appropriately switched to intravenous ampicillin on day 2, with the dosage adjusted to 2 g every 6 hours based on the preadmission creatinine clearance, followed by oral amoxicillin (1500 mg, daily). Outcomes: The abscess disappeared without drainage on day 39, and the patient remained disease-free without recurrence or sequelae during a 6-month follow-up period. Lessons: SDSE can cause a solitary primary iliopsoas abscess, which can be successfully treated with an appropriate dose of antimicrobials without draining the abscess.


Introduction
Iliopsoas abscess is a collection of pus in the iliopsoas compartment and is classified as primary iliopsoas abscess, mainly caused by hematogenous seeding from a distant site, and secondary iliopsoas abscess, which occurs due to underlying diseases, including adjacent vertebral osteomyelitis. [1]Staphylococcus aureus is the most common pathogen isolated from over 88% of patients with primary iliopsoas abscess; meanwhile, Streptococcus dysgalactiae subspecies equisimilis (SDSE) YF and HT contributed equally to this work.

Written informed consent was obtained from the patient and her daughter for publication of this case report and the accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.
The authors have no funding and conflicts of interest to disclose.
has rarely been isolated from with primary iliopsoas abscess. [1]Moreover, SDSE, belonging to Lancefield groups C and G, can cause skin and soft-tissue infections, intraabdominal and epidural abscesses, and infective endocarditis in immunocompromised patients, such as those with diabetes mellitus. [2]revious cases of iliopsoas abscess caused by SDSE have been accompanied by other disseminated foci. [3,4]o the best of our knowledge, there has been no case report of solitary primary iliopsoas abscess caused by SDSE; therefore, the pathogenesis and appropriate therapeutic strategy for such patients remain uncertain.Here, we describe an immunocompetent patient with a solitary iliopsoas abscess caused by SDSE infection and the treatment regimen administered to the patient.

Case
An 85-year-old Japanese woman was admitted to our hospital because of aggravating right hip pain and fever after performing an exercise of lifting both legs while lying on the back 2 days ago.Her height and weight at the first visit were 153 cm and 66.4 kg, respectively (body mass index: 28.4 kg/ m 2 ).She had undergone bilateral total hip arthroplasty for hip osteoarthritis 20 years ago and had a history of mitral regurgitation, hypertension, atrial fibrillation, and chronic kidney disease with 22.9 mL/min of creatinine clearance (CrCL).She took apixaban (5 mg, daily) as her regular medication for atrial fibrillation.
On admission, she was alert and her vital signs were as follows: body temperature, 38.0 °C; blood pressure, 105/45 mm Hg; heart rate, 76 beats/min; respiratory rate, 16 breaths/ min; and oxygen saturation, 90% on ambient air.Physical examination showed that passive extension of the right hip joint caused pain and she had bilateral leg edema without an apparent wound.Laboratory findings showed an elevation of White blood cell (WBC) count with a left shift of neutrophils, a high C-reactive protein (CRP) level, a high erythrocyte sedimentation rate (ESR), normocytic anemia, and renal dysfunction (Table 1).Hip computed tomography (CT) revealed a right iliopsoas abscess (iliac fossa abscess) sized 1.9 cm × 6.0 cm × 5.2 cm without disseminated foci in other sites (Fig. 1A  and B).Transthoracic echocardiography showed no evidence of infective endocarditis.
The patient was administered intravenous cefazolin (0.5 g every 12 hours), as initially methicillin-susceptible S aureus was suspected as the causative agent of the iliopsoas abscess.Two blood cultures were obtained, and after incubating the cultures for 11 hours, gram-positive cocci arranged in long chains were detected using the BacT/Alert system (bioMérieux, Marcy I'Etoile, France) (Fig. 2).The isolates were grown on 5% sheep blood agar (Nihon Becton Dickinson, Tokyo, Japan) and showed β-hemolysis with Lancefield group G antiserum (Kanto Chemical Co., Inc., Tokyo, Japan).The pathogen was identified as S dysgalactiae with a high score value (>2.0) using matrixassisted laser desorption/ionization (MALDI Biotyper; Bruker Daltonik GmbH, Bremen, Germany).
The patient was diagnosed with a solitary iliopsoas abscess caused by SDSE.The isolates were found to be susceptible to penicillin using the VITEK 2 system (bioMérieux) (Table 2).The antimicrobial treatment was appropriately switched to intravenous ampicillin (2 g every 8 hours) on day 2 when the acute kidney injury (AKI) recovered to 22.8 mL/min, as measured using the Cockcroft-Gault formula.The patient became apyrexial on day 3, with gradual relief of right leg pain.Two sets of follow-up blood cultures on day 4 were negative for SDSE.Drainage of the abscess was not performed, considering the risk of hematoma due to apixaban.A follow-up CT on day 16 showed a shrinking abscess (Fig. 1C and D).The abscess disappeared without drainage on day 39 after intravenous administration of ampicillin for 10 days, followed by oral amoxicillin (1500 mg, daily) for 4 weeks.The patient remained disease-free without recurrence or sequelae during a 6-month follow-up period.

Discussion
This report illustrates 2 clinical themes: the potential of SDSE to cause solitary iliopsoas abscess and the appropriate antimicrobial strategy to treat such patients with or without draining the iliopsoas abscess.SDSE, typically β-hemolytic, may express Lancefield group C or G antigens. [2]The mortality rate of SDSE bacteremia has been reported to be 15% to 18%, [5] whereas that of S aureus bacteremia is 34%. [6]Because the iliopsoas muscle has enriched blood supply, a bloodstream infection can easily affect this muscle. [1]Primary iliopsoas abscess has been reported to be caused  by S aureus mainly (88%); in contrast, Streptococcus species have rarely been reported to cause primary iliopsoas (4.8%). [7] PubMed search using the Medical Subject Headings "Psoas abscess" and "Streptococcus dysgalactiae" with the Boolean operator "AND" found only 2 articles in English each describing a case of iliopsoas abscess caused by S dysgalactiae (Table 3).[3,4] We excluded case reports regarding infections caused by group C or G streptococci, as they may include not only S dysgalactiae but also other streptococci, such as S. anginosus.In both the cases of iliopsoas abscess we found, [3,4] blood cultures were positive for SDSE, and there were other sites of SDSE infection in addition to SDSE iliopsoas abscess.A retrospective, multicenter surveillance study in Japan reported 6 cases of iliopsoas abscess with SDSE bacteremia.[8] All cases included other sites of infections: 2 cases with vertebral osteomyelitis and 1 case with each of the following co-infections: cellulitis, septic arthritis, vertebral osteomyelitis pyogenic lymphadenitis, and vertebral osteomyelitis and urinary tract infection.To the best of our knowledge, the present study is the first case report on solitary iliopsoas abscess caused by SDSE.Thus, SDSE should be considered a causative microorganism even in cases of solitary iliopsoas abscess.Iliopsoas abscesses are classified as primary, caused by bacteremia, or secondary, caused by bacterial spread from an infected organ.[1] The patient was diagnosed with a primary iliopsoas abscess because the 2 sets of blood cultures of the patient were positive for SDSE and the CT scan did not find any infected organ. The mt common causative microorganism for a primary iliopsoas abscess is S aureus, whereas pathogens responsible for a secondary iliopsoas abscess are more commonly from the gastrointestinal and genitourinary tracts, such as Escherichia coli, Klebsiella species, and Streptococcus species.[9] The mortality rate of patients with gas-forming iliopsoas abscesses, in which gram-negative rods are predominant, is higher than that of patients with non-gas-forming iliopsoas abscesses, in which

Fuchita et al. • Medicine (2024) Medicine
gram-positive cocci are Additionally, advanced age, WBC count, platelet count, blood urea nitrogen, creatinine, potassium, and secondary iliopsoas abscesses have been reported to be associated with the mortality rate of patients with iliopsoas abscesses. [10]Tabrizian et al [11] reported that patients with bacteremia and small abscess (<3.5 cm) responded well to antimicrobial therapy alone.In gas-forming or non-gas-forming cases without contraindication for surgical intervention, surgical intervention, including percutaneous drainage or operation, remains the first choice for treating iliopsoas abscesses. [10]In the cases where S dysgalactiae was the causative agents of iliopsoas abscess, [3,4] the patients underwent drainage or aspiration in addition to antimicrobial administration (Table 3).However, considering the contraindication of CT-guided drainage and the result of follow-up blood cultures, our patient was treated with intensive intravenous ampicillin for 10 days, followed by 4 weeks of oral amoxicillin administration.As a result, the abscess disappeared without drainage on day 39.This suggests that large solitary iliopsoas abscesses caused by SDSE bacteremia can be treated with antimicrobial therapy alone without needing to drain the abscess.The high blood supply to the iliopsoas muscle may have contributed to our case's excellent response to antimicrobial therapy.Generally, inflammation markers in patients with myalgia and fever should be evaluated, [12] as inflammation markers such as WBC, CRP, and ESR are often elevated in patients with iliopsoas abscesses. [1]The duration of intravenous antimicrobial administration is generally 2 weeks, followed by oral antimicrobials for 4 to 6 weeks, depending on inflammatory marker levels, clinical improvement, and follow-up CT results. [13]The patient was treated with intravenous ampicillin for 10 days and oral amoxicillin for 4 weeks, according to WBC count, CRP, ESR, clinical improvement, and follow-up CT findings, without draining the abscess.
The antimicrobial dosage adjustment based on creatinine prior to admission may contribute to the success of the treatment.One-fifth of patients with acute infections develop AKI on admission; however, more than 50% of the patients with AKI have a resolution of renal injury by 48 hours. [14]Therefore, during that time, the dose of beta-lactam drugs with a comprehensive safety zone should not be reduced. [14]Antimicrobials with a wide safety margin, such as β-lactam and β-lactam/βlactamase inhibitor combinations, allow dose adjustments to be deferred until 48 hours after the initiation when the patient's renal function is better characterized. [14]In addition, appropriate antimicrobial treatment for community-onset bacteremia within 48 hours is negatively correlated with the mortality rate. [15]Our patient was treated with cefazolin as an empirical therapy with the dosage designed based on the CrCL on admission.However, the antimicrobial was switched to ampicillin the following day, with the dosage adjusted based on the preadmission CrCL.In our case, CrCL on day 2 improved from 11.4 to 22.8 mL/min; therefore, the patient was diagnosed with reversible AKI and did not require the readjustment of the antimicrobial dosage.Clinicians should pay attention to the information on preadmission CrCL for appropriate antimicrobial administration.
A limitation includes that SDSE was not directly proven as the causative organism of the iliopsoas abscess because of the high risk of hematoma.
In conclusion, SDSE can cause a solitary iliopsoas abscess.Clinicians should remain aware that the abscess caused by SDSE bacteremia can be treated with an appropriate dose of antimicrobials and may not require drainage.A = alive, CRP = C-reactive protein, F = female, M = male, WBC = white blood cell.

Figure 1 .
Figure 1.(A and B) Computed tomography (CT) on admission revealed a solitary right iliac fossa abscess sized 1.9 cm × 6.0 cm × 5.2 cm (red arrow and arrowhead) (A, sacroiliac level; B, lower sacral level).(C and D) A follow-up CT on day 16 showed a shrinking abscess (yellow arrow and arrowhead) (A, sacroiliac level; B, lower sacral level).

Table 1
Laboratory data.

Table 2
Antimicrobial susceptibility test results.
MIC = minimum inhibitory concentration, R = resistant, S = susceptible.* Interpreted according to the Clinical and Laboratory Standards Institute criteria (Documents M100-Ed32).